The differences in color in the sexes of birds are classified by Darwin as follows: (1) when the males are ornamented exclusively or in a much higher degree than the females; (2) when both sexes are highly ornamented; (3) when the female is more brightly colored. A few examples of each sort may be chosen for illustration. “In regard to color, hardly anything need here be said, for every one knows how splendid are the tints of many birds, and how harmoniously they are combined. The colors are often metallic and iridescent. Circular spots are sometimes surrounded by one or more differently shaded zones, and are thus converted into ocelli. Nor need much be said on the wonderful difference between the sexes of many birds. The common peacock offers a striking instance. Female birds of paradise are obscurely colored and destitute of all ornaments, whilst the males are probably the most highly decorated of all birds, and in so many different ways, that they must be seen to be appreciated. The elongated and golden-orange plumes which spring from beneath the wings of the _Paradisea apoda_, when vertically erected and made to vibrate, are described as forming a sort of halo, in the centre of which the head ‘looks like a little emerald sun, with its rays formed by the two plumes.’” Male humming-birds are almost as splendidly colored as are the birds of paradise, some having the feathers modified in a truly extraordinary way. “Almost every part of their plumage has been taken advantage of, and modified; and the modifications have been carried, as Mr. Gould showed me, to a wonderful extreme in some species belonging to nearly every subgroup. Such cases are curiously like those which we see in our fancy breeds, reared by man for the sake of ornament: certain individuals originally varied in one character, and other individuals of the same species in other characters; and these have been seized on by man and much augmented—as shown by the tail of the fantail pigeon, the hood of the jacobin, the beak and wattle of the carrier, and so forth. The sole difference between these cases is that in the one the result is due to man’s selection, whilst in the other, as with humming-birds, birds of paradise, etc., it is due to the selection by the females of the more beautiful males.”